1. Field of the Invention
This invention is directed to a method for oxidizing a flue gas desulfurization waste product to yield a useful end product and to the oxidized product produced by the process.
2. Description of the Prior Art
The combustion of coal in power plants generates large volumes of solid and gaseous pollutants, including fly ash and sulfur dioxide. Sulfur dioxide has been identified as the culprit in acid rain and substantial effort has been directed to reducing or eliminating the amount of sulfur dioxide discharged into the atmosphere by coal-fired boilers. One route taken to accomplish this end is to treat the off-gases of the boiler (the flue gas) to remove the sulfur dioxide therefrom by reacting it with a calcium-containing compound to form calcium-sulfur compounds. These calcium-sulfur compounds have then been disposed of by ponding, if the compounds are in th form of a sludge, or as land-fill if the compounds are dry.
The most common flue gas desulfurization (FGD) process is known as the "wet process." In that process the sulfur dioxide-containing flue gas is scrubbed with a slurry containing calcium hydroxide or another hydrated alkaline absorbent. The scrubbing takes place, for example, in a tower in which the gas flow is countercurrent to and in intimate contact with a stream of descending slurry. The slurry may flow over packing or trays within the tower. The spent slurry product of this FGD process contains both calcium sulfite and calcium sulfate. It has been found to be advantageous to convert the calcium sulfite in the product to calcium sulfate by bubbling air or other oxygen-containing gas through the slurry. Such a process is shown, for example, in U.S. Pat. No. 4,024,220, which issued May 17, 1977. The calcium sulfate product may be used in the production of wall board. Other U.S. patents which disclose various features of the "wet process" are: U.S. Pat. Nos. 4,371,508 4,366,132 4,328,195 4,069,300 4,024,220 3,985,860.
Another type of FGD system produces a dry waste product. A typical dry FGD system comprises: (1) a pre-dust collector for removing fly ash, consisting of an electrostatic precipitator, cyclones or other dedusting equipment; (2) a sulfur scrubbing system, consisting of a liquor preparation system for preparing milk of lime slurry absorbent and a spray drying tower where the absorbent is injected into the flue gas to absorb sulfur dioxide (SO.sub.2) and (3) a final dust collecting system.
In the dry process FGD system, sulfur dioxide-containing flue gas is introduced into a spray drier in which a slurry of a lime-containing liquid is sprayed into contact with the flue gas. Reactions occur between the lime and the sulfur dioxide in the flue gas, while the liquid of the slurry is evaporated, yielding an essentially dry powder containing calcium-sulfur compounds. Such a process is shown, for example, in U.S. Pat. No. 4,197,278, which issued Apr. 8, 1980. The dry powder product contains both calcium sulfite and calcium sulfate and it has been suggested that this powder be reslurried and then subjected to oxidation by bubbling air therethrough to convert the calcium sulfite to calcium sulfate in accordance with the teachings of the "wet process" prior art as described above.
As described above, a characteristic of the dry FGD system is that the liquid phase of the fresh absorbent when injected into the spray-drying tower, is very nearly completely evaporated when the flue gas leaves the tower. The dry, spent absorbent of fine-grained particles is first entrained in the desulfurized flue gas and is then partially separated and collected in the bottom section of the spray-drying tower, with the remaining entrained portion removed from the flue gas flow in a final dust-collecting system, usually a bag filter.
The fresh or make-up absorbent is often supplied to the FGD system in dry form (burned lime), where it has to be converted into a liquor, by slaking in water.
The system for preparing and injecting the liquid absorbent comprises:
1. A repulper for mixing the solid absorbent into a liquor or a slurry; and the repulper is often equipped with a screen to take out coarse materials not suspended properly;
2. Pumps for moving the slurry within the system and from the repulper to the spray drying tower, and
3. One or more injection nozzles, mounted on the spray drying tower for dispersing the absorbent liquor into the flue gas stream.
Part of the collected dry, spent absorbent is repulped with the fresh liquor-absorbent to improve the utilization of the absorbent since unreacted absorbent does pass through the spray dryer. This repulping is normally performed in a separate repulping vessel. The recycle ratio is typically 3:1; i.e., three parts of recycled spent absorbent to one part of raw feed.
The following table sets forth typical characteristics of the FGD product of the dry FGD system where milk of lime is used as the absorbent spraying into the flue gas:
TABLE I ______________________________________ Chemical Composition Weight % ______________________________________ CaSO.sub.3.1/2 H.sub.2 O 40-70 CaSO.sub.4.2H.sub.2 O 5-15 CaCO.sub.3 0-10 Ca(OH).sub.2 5-15 CaCl.sub.2.xH.sub. 2 O (x = 1, 2 or 4) 0-10 Fly ash 5-25 ______________________________________
Particle size:
100% smaller than 100 microns (150 mesh) PA1 70-90% smaller than 50 microns PA1 SiO.sub.2 PA1 Fe.sub.2 O.sub.3 PA1 Al.sub.2 O.sub.3 PA1 Na.sub.2 O+K.sub.2 O
The calcuim sulfite is directly formed when milk of lime reacts with gaseous sulfur dioxide. In the temperature range of 70.degree.-150.degree. C., at which a spray-drying tower normally operates in FGD systems, and because of the relatively low oxygen levels of 3-5% in the flue gas, only a small part of the calcium sulfite is oxidized into the sulfate form.
Part of the milk of lime does not react at all (serving as the excess absorbent required for a high percentage removal of SO.sub.2), while another part of the milk of lime is carbonated due to the high CO.sub.2 level of the flue gas. Most coals also contain some chlorine, which becomes volatile during combustion and which also reacts with the milk of lime in the spray-drying tower forming CaCl.sub.2.
The flue gas also contains an entrained fly-ash component which must be dealt with. Fly-ash typically contains the following compounds:
The suggestion has been made in the prior art that the dry calcium sulfite/calcium sulfate-containing powder be oxidized in the dry state and the product obtained thereby, having a high calcium sulfate content, be utilized as an additive for cement or concrete in which it functions as a retardant.
Accordingly, there have been efforts in recent years to develop an oxidation process for treating the dry powder calcium sulfite/calcium sulfate product of the spray drying process of flue gas desulfurization to obtain an oxidized product having a high calcium sulfate content which can be utilized in the known markets. Certain of these efforts are described in the following publications:
West German Application DE No. 3015977 (Holter), published Nov. 11, 1981, is directed to a flue gas desulfurization process and system for treating boiler off-gases in which spent absorbent containing a mixture of CaSO.sub.3 and CaSO.sub.4 is separated from the flue gas and injected into a region of the boiler combustion chamber in which an excess of air is present and the temperature therein is not substantially more than 800.degree. C., and is preferably less than 800.degree. C., to bring about oxidation of CaSO.sub.3 to CaSO.sub.4.
European patent application No. 74,258 (A/S Niro Atomizer), published Mar. 16, 1983, is directed to a process for treating spent absorbents resulting from desulfurization of flue gas in a spray drying-absorption process, the absorbents containing substantial amounts of sulfite. The sulfite is oxidized into sulfate by heating the spent absorbent in the presence of an oxygen-containing gas at temperatures in the range of 350.degree.-600.degree. C.
West German patent application DE No. 3135200 (L and C Steinmuller GmbH), published Mar. 17, 1983, is directed to a process for the treatment of the end products of flue gas desulfurization, the flue gas desulfurization being effect by absorption of sulfur oxides in a spray drier with a calcium-containing suspension or in a reactor with dry granular calcium-containing absorbent. The end products of the flue gas desulfurization comprise the compounds, calcium sulfite hemihydrate and/or calcium sulfite and/or calcium sulfate dihydrate and/or calcium sulfate hemihydrate and/or calcium sulfate, with the balance the unreacted absorbent Ca(OH).sub.2. The calcium sulfite-containing end products are treated by dry oxidation with air at temperatures between 400.degree. and 800.degree. C. and, preferably, between 400.degree. and 600.degree. C.
Experience in thermal processing (roasting, calcination) at high temperatures (400.degree.-1000.degree. C.) of various fine grained materials (for example, lime mud, cryolite, pyrites) in fluidized bed reactors has shown that it is sometimes possible to achieve under certain conditions agglomeration sintering of the fine particles into pellets with a diameter of several millimeters.
In U.S. Pat. No. 2,465,410, issued Mar. 29, 1949, there is disclosed a process for calcining lime bearing sludge such as paper mill causticizing sludge, water softening sludge, sludge from first carbonation or other lime sludge producing stations in sugar factories. The sludge solids are provided with a proper quantity of an adhesive substance (soda being an example) which softens and becomes adhesive at elevated temperature to cause particle agglomeration so that the calcined product in in the form of essentially dust-free nodules or pellets.